An alternator, as in an automotive application, provides a voltage to the vehicle's load circuits. The voltage supplied by the alternator is subject to wide variations and transients that can be harmful to the circuits. Typically, a voltage regulator is used to provide a constant voltage. The voltage regulator takes a voltage directly from the alternator, or battery, and changes it to the desired voltage while suppressing any transients.
Typically, in automotive applications, the voltage is reduced from a nominal vehicle level, typically nine to sixteen volts, to a desired voltage level by a series regulator. In a series regulator the current used by any load circuit passes through the regulator. The product of the voltage across the regulator and the current through the regulator is the power dissipated by the regulator. This power dissipation by the regulator causes the temperature of the regulator to rise, thereby limiting the amount of current it can deliver to a load circuit at a fixed voltage. The package of the regulator limits the amount of heat that can be dissipated at a steady state fixed rate from the regulator, which is typically an integrated circuit die. As the power dissipation increases, the temperature of the regulator package and integrated circuit die also increase. Manufacturer specifications determine how much power can be reliably dissipated by the regulator package before the integrated circuit is adversely affected. Typically, at power levels above the manufacturer's specifications the integrated circuit die becomes separated from the regulator package, resulting in failure of the voltage regulator.
The cost of an integrated circuit depends on the cost of the integrated circuit die, or inner workings of the circuit, and the cost of the integrated circuit package, or the outer housing of the integrated circuit. As would be expected, the package for an integrated circuit capable of dissipating high power is expensive.
Methods exist for overcoming the limitations of the integrated circuit package power dissipation capabilities. One method is to employ a heatsink on the integrated circuit package thereby extending the power dissipation capability of the integrated circuit package. Another method is to employ a transistor that is external to the integrated circuit for dissipating the power. This method reduces the amount of power that must be dissipated by the integrated circuit, and allows the voltage regulator to be provided in a less expensive package. However, the regulator circuit becomes more difficult to design because the external components adversely affect the stability of the integrated circuit. Additionally, because of the external components, the package cost of the system integrated circuit is increased.
On the one hand, the output voltage must be kept at a specific value, five volts for example. High input voltages to the integrated circuit will require maximum power dissipation and it would be beneficial to reduce the voltage externally before it reaches the integrated circuit. On the other hand, the regulator must be capable of operating at low input voltages in which case it becomes undesirable to have an external power drop before the regulator integrated circuit. Having to simultaneously meet these two diametrically opposing conditions exemplifies how difficult it is to control the power dissipation in a voltage regulator integrated circuit.